Monoethylene glycol (MEG), often referred to as ethylene glycol, is an important compound that is often used in anti-freeze and in the production of certain polymers, such as polyester and polyethylene terephthalate, which is commonly used in plastic bottles for soft drinks. Ethylene glycol may be produced by the reaction of ethylene oxide and water. Various byproducts, such as di-ethylene glycol (DEG), tri-ethylene glycol (TEG), etc., are often co-produced using this synthesis method, so that yields of monoethylene glycol may be lower than desired. As used herein, the expression “ethylene glycol” without any prefix is meant to encompass monoethylene glycol, unless otherwise stated or is apparent from its context.
Another method of forming ethylene glycol is from ethylene carbonate (EC). Ethylene carbonate may be converted to dimethyl carbonate (DMC) and ethylene glycol in the presence of methanol in a transesterification reaction. Using such reaction method, the yield of ethylene glycol is much higher, with less undesirable byproducts being produced. Additionally, the product dimethyl carbonate produced in such reaction is useful as an oxygenate additive in fuel and in the production of Bisphenol A, which is commonly used in making polycarbonate plastics and epoxy resins.
The ethylene carbonate used in producing ethylene glycol and dimethyl carbonate may be prepared from ethylene oxide in a carbonation reaction. The carbonation of ethylene oxide with carbon dioxide (CO2) yields ethylene carbonate. The use of carbon dioxide as a reactant may be particularly desirable due to the increased emphasis presently placed on minimizing CO2 emissions.
To yield ethylene glycol in the transesterification reaction, a purified source of ethylene carbonate is typically used. Recently, however, integrated processes have been developed that utilize ethylene oxide and CO2 in a first carbonation stage to yield a crude or unpurified ethylene carbonate product, which is then used in a second transesterification stage wherein the ethylene carbonate product is converted to ethylene glycol and dimethyl carbonate.
Although an integrated process eliminates the need for a purified ethylene carbonate source, one of the issues with an integrated process is the effect of the catalyst used in the carbonation reaction on the transesterification reaction.
Homogeneous catalysts are often used for the carbonation reaction. Such homogeneous catalysts may include potassium and quaternary ammonium halide salts, such as those described in U.S. Pat. No. 7,084,292. Without purification of the ethylene carbonate product to remove the homogeneous catalyst, the homogeneous catalyst is carried with the ethylene carbonate product into the reaction zone used for transesterification. For ion exchange resins (IER) catalysts, which are often used in the transesterification reactions, the IER catalyst may tend to absorb the halide ions of the homogeneous carbonation catalysts. Such absorption of halide ions of the homogeneous carbonation catalyst may tend to decrease the effectiveness of the IER transesterification catalyst.
Accordingly, improvements are needed to provide an integrated carbonation/transesterification process for the production of ethylene glycol and/or dimethyl carbonate, and similar products, which overcomes these and other issues.